In most use cases I can think of the generated data is used only in a closed (testing) environment and won't be visible to web crawlers and spambots. Many developers like me use these libraries for prototyping different UI views for their apps.
The bigger problem, in my experience, is when you accidentally spam people yourself, while testing some function that somehow generates an outgoing email. I think almost everybody that's ever worked on code that sends emails has - at one time or another - ran a job that somehow wound up sending 6,712,374 "test" messages to every user in the production database. :-) Dummy addresses are better, but it's better still if they use @example.com so you know there's no way they'll actually get delivered to anybody.
The bigger problem, in my experience, is when you accidentally spam people yourself, while testing some function that somehow generates an outgoing email. I think almost everybody that's ever worked on code that sends emails has - at one time or another - ran a job that somehow wound up sending 6,712,374 "test" messages to every user in the production database. :-) Dummy addresses are better, but it's better still if they use @example.com so you know there's no way they'll actually get delivered to anybody.